I don't think anything but money is the deciding factor when it comes to whether or not the NHL views a team as being successful.
I still think Vegas will make a ton of money. Unless another sport moves there, it'll be the only of the big four sports in a town fanatical about sports gambling and known as a place to go to have a good time.
While I doubt you're gonna find many people in Vegas who have tens of thousands of posts on a message board regarding the nuances of hockey, there will be fans. They will spend money. The NHL will view it as a success.
Agreed on teams like Carolina though. When a team is in as bad financial trouble as Carolina is with their owner, it's odd the NHL opted to expand before relocating.